The goal of this investigation is to examine the genetic events underlying sex differentiation. One group of studies will focus on the production and characterization of mosaic mice in which the sex chromosome balance is experimentally altered. The two initial objectives of these studies are to produce mosaic mice whose sex karyotypes are predetermined, and to develop methods for assaying the tissue distribution of karotypically male and female cells in neonatal mosaic mice. On this basis, it should be possible to analyze systematically the interactions between germ cells and their embryonic environment in the course of gonad development, and in particular to study in detail the perturbation of this development that occurs in sexually mosaic or aneuploid fetuses. The other aspect of this investigation concerns the effects of these factors on the alternation of normal gonad development that leads to the formation of teratomas. Finally, these interactions will be examined directly by following the development of teratma-derived cells introduced into normal blastocysts.